A radiotherapeutic apparatus is well known, and consists of a source of radiation which emits a beam of radiation that is directed toward a patient in order to destroy or otherwise harm cancerous cells within the patient. Usually, the beam is collimated in order to limit its spatial extent to a desired region within the patient, usually the tumour or a sub-section of the tumour, and to avoid irradiating nearby healthy and/or sensitive tissue. The source can be a linear accelerator for high-energy (MV) X-radiation, or an isotopic source such as Co-60. The source is often mounted on a rotatable gantry, so as to be rotated around the patient in order to irradiate the desired region from a number of different directions, thereby reducing the dose applied to healthy tissue around the desired region. The collimator can move to change the shape of the beam as the source rotates, in order to build up a complex dose distribution for tumours with more challenging shapes and/or which are located near to sensitive areas. An electronic portal imaging device (EPID) may be mounted to the gantry on the opposite side to the source so as to receive the beam once it has been attenuated by passage through the patient; this device produces an image which can be used for correctly aligning or calibrating the system, as well as for assessing the patient's location and the placement of the radiotherapeutic treatment.
Incorporating real-time image guidance into radiotherapy can improve tumour targeting accuracy, enabling better avoidance of critical structures and reducing side effects. Such guidance is of particular benefit if a non-ionizing imaging technique such as MRI (magnetic resonance imaging) is employed. Work is currently being undertaken to integrate a linear accelerator with an MR scanner; integrating high-quality MRI with a linear accelerator (creating an “MR Linac”, or MRL) allows tissue to be tracked online, and therapeutic radiation beams can be guided to their targets (which may be moving and deforming, such as when the patient breathes) with sub-millimeter precision during treatment.
Such is the precision with which treatment is to be applied to a patient, it is necessary to align the radiotherapeutic beam with the magnetic field of the MR scanner with a very high degree of accuracy before the patient can be positioned accurately with respect to the apparatus and the radiotherapeutic treatment begun; this degree of accuracy necessitates taking into account mechanical effects, such as sagging or deflection of elements of the apparatus due to gravity (which can vary according to the rotational position of the gantry) such as the source and the EPID or other imaging device, variations in the alignment of the source so that the beam is not directed perpendicularly to the EPID, and other factors which cause the beam, or its trajectory as the gantry rotates, to deviate from the intended path. In linear accelerators used for radiotherapy, cone-beam computed tomography (CBCT) techniques which employ phantoms have been utilised to address the issue of beam alignment (see, for example, J. Chetley Ford, Dandan Zheng and Jeffrey F Williamson. “Estimation of CT cone-beam geometry using a novel method insensitive to phantom fabrication inaccuracy: Implications for isocenter localization accuracy.” American Association of Medical Physics, Med. Phys. 38 (6), June 2011, 2829-2840.; Weihua Mao, Louis lee and Lei Xing. “Development of a QA phantom and automated analysis tool for geometric quality assurance of on-board MV and kV X-ray imaging systems”. Med. Phys. 35 (4), April 2008, 1497-1506; and Youngbin Cho, Douglas J. Moseley, Jeffrey H Siewerdsen and David A Jaffray. “Accurate technique for complete geometric calibration of cone-beam computed tomography systems”. Med. Phys. 32 (4), April 2005, 968-983), but conventional techniques are not readily transferable to MRLs, firstly because there is restricted space in the bore of the MR scanner (so restricted that it is not feasible to use lasers or other light projection systems to assist in aligning the beam correctly), and secondly because phantoms visible to X-ray imagers are not usually visible to an MR scanner. Moreover, conventional approaches are incapable of accurately aligning both the beam and the beam imaging device, accounting for all the factors affecting their deviation from the ideal, nor can they be used for the alignment of associated equipment, such as a beam collimator, nor to align the beam so that its profile (on the beam imaging device, or EPID, and hence on the patient) is symmetrical (referred to hereinafter as “beam symmetry”).